Recently, researchers at the New Zealand Institute of Science (GNS Science) produced a detailed map of the bottom of Lake Rotorua and revealed signs of hydrothermal activity, including a strange “magnetic anomaly” along with numerous small craters covering the bottom.

It’s like finally being able to put on your glasses and read the small print,” GNS Science principal scientist Cornel de Ronde said in an interview with LiveScience.

In total, the researchers surveyed an area of ​​54 square kilometres, or about 68 per cent of Rotorua. Their primary method was to use multibeam sonar to determine the depth and shape, or bathymetry, of the lake bottom.

This revealed the first sign of hydrothermal activity: thousands of “pits” up to 51 meters in diameter covering the bottom of the lake. Researchers say these small craters are left over from the release of gas and hot water and still emit weak heat currents. According to De Ronde, the lake is generally very cold, with 13.8 degrees Celsius near its bottom.

Magnetic scans revealed another sign: magnetic anomaly. Volcanic rocks often contain magnetite, which, as the name suggests, is highly magnetic. But hot water can transform magnetite into pyrite, another mineral with almost zero magnetic signal.

Lake Rotorua Map / Photo: GNS Science

Normally when you run a magnetometer over volcanic rocks you get very positive anomalies, but in this case we get negative anomalies probably due to very low magnetic susceptibility.
de Ronde told LiveScience.

The researchers concluded that the “pits” and their heat fluxes, as well as dead zones of magnetic intensity, were strong evidence that the geothermal systems supporting parts of the lake were still active.

• But that’s not all. The researchers also found that the magnetic anomaly overlaps with a “thin heat flow anomaly,” suggesting another hydrothermal system may exist.
• What’s more, they found evidence of an ancient river near the shore that existed before the formation of the lake around 65,000 years ago.

All this shows that the ancient volcano, although dormant, continues to affect the lake.